feat: add file menu actions for agent/workspace workflows

[Dimillian]

Motivation

• Expand the File menu so users can create agents and workspaces from the app menu instead of only closing the window.
• Reuse existing frontend handlers for agent/worktree/clone/workspace flows so menu actions follow the same behavior as UI shortcuts.
• Emit menu events from the Tauri backend so the frontend can subscribe and react in a platform-appropriate way.

Description

• Added new File menu items in src-tauri/src/lib.rs for New Agent, New Worktree Agent, New Clone Agent, Add Workspace..., and Settings..., and emit corresponding events (menu-new-agent, menu-new-worktree-agent, menu-new-clone-agent, menu-add-workspace, menu-open-settings).
• Added subscription helpers in src/services/events.ts (subscribeMenuNewAgent, subscribeMenuNewWorktreeAgent, subscribeMenuNewCloneAgent, subscribeMenuAddWorkspace, subscribeMenuOpenSettings) that listen for the emitted menu events.
• Updated src/App.tsx to import the new subscriptions, refactored the existing add-agent/add-worktree/add-clone/add-workspace handlers to useCallback, and subscribed to the new menu events to invoke the existing flows (handleAddAgent, handleAddWorktreeAgent, handleAddCloneAgent, handleAddWorkspace, handleOpenSettings).
• Ensured the existing CMD+N/new-agent shortcut continues to call the unified handleAddAgent handler so keyboard and menu actions are consistent.

Testing

• Ran npm run lint which completed but produced a TypeScript version warning about the installed TypeScript being newer than the supported range.
• Ran npm run typecheck and fixed type issues; tsc --noEmit completed successfully.
• No new automated unit tests were added for the menu wiring in this change.

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Codex Task